May, June & September: The Golden Shoulder Months
Late May, June, and September offer the best balance of mild 18°C–24°C weather, lower hotel rates than July–August, and up to 17–18 hours of daylight in June (sunset after 22:00 in Edinburgh and Skye!). May and September are also largely free of Highland midges in Scotland.
August (Edinburgh Fringe) & December (Festive Markets)
In August, Edinburgh hosts the world's largest arts festival (Edinburgh Festival Fringe & Royal Edinburgh Military Tattoo — book hotels 6+ months early). From mid-November to Christmas, Bath, York, Edinburgh, and London's West End glow with festive markets and ice rinks.